The present invention relates to a two-way paging system with an answer-back function and, more particularly, to a two-way paging system using an orthogonal detection/reception scheme.
With recent advances in data communication techniques and compact circuit techniques, a two-way paging system has been developed as a private branch communication system, which can transmit an incoming call answer-back signal, a message, and the like from a paging receiver side.
A two-way paging receiver used for a private branch paging service inevitably has a low-power transmission circuit. For services in a narrow area, e.g., a private branch paging service, the paging receiver is only required to perform short-distance transmission. Therefore, the realization of such a two-way paging system is relatively easy.
In some two-way paging systems, the transmission and reception frequencies are set to be equal to each other in consideration of the effective use of waves. In such a case, contention between reception and transmission is avoided by transmitting answer-back signals for acknowledging reception at predetermined time intervals.
In some other paging receivers, the orthogonal detection/reception scheme requiring no RF and IF filters has recently been used to achieve a reduction in size. In the orthogonal detection/reception scheme, the line frequency and the local oscillation frequency are set to be equal to each other, and beats of the reception frequency and the local oscillation frequency are extracted by a mixer. The beats are input to a low-pass filter to obtain only a baseband signal. The baseband signal is amplitude-limited by a limiter circuit. In addition, the resultant signal is demodulated to obtain an demodulated signal.
In the orthogonal detection scheme, since the local oscillation frequency and the line frequency coincide with each other, the IF becomes zero and no image frequency is present. Therefore, a harmonic amplifier and an IF amplifier do not require any high-selectivity filters for attenuating the image frequency. In addition, since the IF is zero, a channel filter for attenuating adjacent channel interfering waves can be constituted by a low-frequency active filter and can be formed on an integrated circuit.
As a conventional two-way paging system of this type, a system disclosed in Japanese Patent Laid-Open No. 61-228735 is known.
In the above-described conventional two-way paging system, since an answer-back operation must be performed without using radio frequencies, a great change in an existing transmission system is inevitably required, resulting in difficulty in constructing a system.
Furthermore, since a portable radio unit in the conventional two-way paging system requires a transmission circuit as well as a reception circuit, the size of the radio unit is much larger than that of a normal paging receiver. This poses a serious problem in portable radio units which are required to be reduced in size and weight.